This invention relates to a method and laser system for correlating the spontaneous emission of photons and thereby quenching the noise due to spontaneous emission in laser fields.
High-precision or high-sensitivity laser interferometers are finding increasing use in the detection of small length or frequency perturbations. Some optical length perturbation sources or length modulation sources are: gravity waves, variations in the index of refraction, temperature fluctuations, and mechanical vibrations. There have been prior attempts to detect extremely small perturbations due to gravitational radiation, but these attempts have been unsuccessful. The frequency of gravitational radiation is typically in the range of 10.sup.-2 to 10.sup.4 Hz, and the strength of gravitational radiation is typically in the range of 10.sup.-17 to 10.sup.-26 (dimensionless amplitude; the ratio of the change in proper distance (.DELTA.L) between two free masses to the nominal length (L)). Gravitational radiation is a result of astronomical events, such as the formation of black holes, supernovae, or rotating binary stars. To detect this radiation, extremely sensitive lasers are required.
There are two standard types of laser interferometers; passive and active laser systems. The sensitivities of both types of laser systems are approximately the same. The signal power for an active laser device is independent of the length of the device, whereas the signal for a passive laser device is a linear function of the arm lengths of the interferometer. Active laser systems have a larger signal than passive laser systems, but are hindered by larger noise sources. These factors result in essentially identical quantum limits for both prior art systems.
Passive laser systems have been used in the prior art to attempt the detection of gravitational radiation. Currently, there is a joint project between M.I.T. and Caltech to build two, five-kilometer long, passive laser interferometers to yield evidence of and information on gravitational radiation. In passive laser systems, generally one laser and a beamsplitter is utilized to produce two perpendicular beams. The length between the beamsplitter and one of the mirrors is fixed (this is the distance traversed by the "reference" or "standard" beam) and the other length (this is the distance traversed by the "signal" beam) is allowed to be influenced by an external force. When there are no external forces, the phase difference (at the photodetector) between the two beams produced by the beamsplitter will be constant. However, an external force, i.e., the effect to be measured, causes a change in the length traversed by the "signal" beam but not that traversed by the "standard" beam. Either length could change as long as it did so in a different manner. In the case of the passage of gravitational radiation, differential change occurs because the gravity wave only affects distances which have a component perpendicular to the direction of the propagation of the wave. Thus the interferometer can be situated such that only the "signal" length has a component perpendicular to the direction of the passing gravitational radiation.
A change in length for a passive laser system is represented by the following equation: ##EQU1## in which ".DELTA.L" is the magnitude of the length modulation of an interferometer arm when affected by a perturbation source, "L" is the original length, "h" is a dimensionless parameter which characterizes the strength of the perturbation source, ".omega." is the frequency of the perturbation source, and "t" is the time. As can be seen from Equation 1, for very small perturbation sources which are characterized by low strengths and low frequencies, the effect of the length modulation (.DELTA.L/L) is very small, and often cannot be detected with the use of prior art passive laser means and devices. Gravitational radiation has not yet been detected in the prior art. Passive laser systems are limited in their sensitivity and thus detection ability due to their small signals and noise present in the laser system. The limiting noise in passive laser systems is generally "shot noise" which is associated with the imperfect conversion of photons to photoelectrons at the photodetector.
Active laser systems, which generally comprise two laser modes or fields, generate a larger signal and thus can feasibly detect minor perturbation sources more easily than passive systems. Active laser interferometers operate on the principle that a laser field will oscillate at a frequency equal to one of the frequencies of the laser cavity. Generally, two lasers in an active system are positioned perpendicular to each other. One of the lasers typically serves as a "reference" or "standard" laser; this reference laser has a fixed cavity length and thus a fixed frequency. When a perturbation source passes the other laser ("signal" laser) there is a change in the cavity length, resulting in a change in the laser frequency. This change in frequency is then detected by beating or heterodyning the signal laser against the stable reference laser. An electronic current is produced which oscillates at the frequency difference between the two lasers; this information is typically resolved by conventional electronics.
The frequency change for an active laser system is represented by the following equation: ##EQU2## in which ".DELTA..nu." is the change in frequency due to the perturbation source, ".nu." is the original frequency, ".DELTA.L" is the change in cavity length, and "L" is the original cavity length. As can be seen from Equation 2, a small change in cavity length results in a relatively large frequency shift. Thus, the cavity length does not need to be as long as with a passive laser system (table-top length compared to hundreds of meters long for a passive system). Therefore, an active laser system, being smaller than a comparable passive laser system, is generally preferable for detection purposes.
Active laser systems, however, have greater environmental disturbances or "noise" than passive systems which limits their sensitivity. Both types of laser systems (passive and active) have noise largely due to mechanical vibrations which tend to move the mirrors, and temperature fluctuations in the lasing medium which tend to change the beam length and thus the length stabilization of the system. Prior art means have been fairly effective in eliminating the effects of these major types of noise.
Shot noise is a primary source of noise in active laser systems; however, a more limiting source of noise in active systems is due to the random nature of spontaneous emission which is intrinsically present in the laser cavity along with the much more dominant stimulated emission. Spontaneous emission noise causes the phase of an active laser system to randomly drift. (Such a drift in a passive system is not considered "noise" because both beams are equally affected and the drift is thus compensated for.) If this random drift has a larger amplitude than the deterministic laser signal, information about the physical parameters of a perturbation source cannot be reliably determined. Attempts in the prior art to quench or eliminate the effects of simultaneous emission noise in active laser systems have been unsuccessful.
Accordingly, it is a primary object of the present invention to provide a method for eliminating the effects of spontaneous emission noise in active laser systems.
Another object of the present invention is to provide improved active laser interferometers having high precision for use in detecting minor perturbations sources, such as gravity waves.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description to follow, taken in conjunction with the accompanying drawing.